1. Field of the Invention
The present invention relates in general to a carbonizer or pyrolyzer, and in particular to a two stage carbonizer.
2. Description of the Related Art
A carbonizer or pyrolyzer is a device that drives off the volatile matter of a fuel such as coal to produce gas, liquid tars and oils, and char consisting of mostly carbon. In the 1970's, FMC Corp. developed a process known as COED that pyrolyzed coal to produce a gas containing H.sub.2, CO, CH.sub.4 and a liquid product for gasoline production. This process was an enhanced version of an earlier process called FormCoke, also developed by FMC Corp. FormCoke also pyrolyzed the coal at about 950.degree. F. to 1000.degree. F. to produce tar and char. The tar was conditioned and then recombined with the char. The mixture was heated to about 1500.degree. F. to produce a coke product. This process was able to produce coke from non-coking coals cleanly. A plant using this process was built in Kemerer, Wyoming and produces around 93,000 tons per year of coke. The difference between the FormCoke and COED processes was that for the COED process, the char product was burned and the resulting flue gas provided heat to pyrolyze the coal.
Other companies have developed devices to produce synthetic methane gas by similar processes however the temperatures were increased to reduce tars and oils. Higher temperatures thermally crack the tars to produce H.sub.2, CO, and CH.sub.4. Blast furnace coke production pyrolyzes the coal at temperatures above the ash softening temperature. The tars produced are sought after for their chemicals. The long chain cyclic hydrocarbons (coal tars) are cracked into various mixtures of paraffins, olefins, and cyclic and aromatic hydrocarbons.
Recently, the idea of pyrolyzing coal at higher temperatures, which is now termed mild gasification, to produce a low Btu gas and a char is under investigation in an advanced pressurized fluidized bed combustion cycle (PFBC). Also, addition of limestone and dolomite have provided sulfur removal characteristics. The low Btu gas is used to increase the temperature of the flue gas from a PFBC to increase cycle efficiency and to burn the char in the PFBC. Currently, several companies are pursuing mild gasification for this purpose. Other companies have also begun to develop advanced PFBC cycles. The main thrust of the new carbonizers or mild gasification development is to produce a gas and char with no tars or oils. The addition of sorbent to the carbonizer enhances the cracking of the long chain hydrocarbon that make up the tars at lower temperatures.
Thus, there is a need for a device which places as much heat as possible into the gas streams entering the carbonizer to drive off volatile matter and reduce tars and oils by thermal cracking. It is desirable to add sorbent to enhance the thermochemical reaction by aiding in the cracking of the tars.